Friction energy adsorption in fiber reinforced composites

Energy absorption of fiber reinforced composite structures is of interest to the automotive industry as their specific energy absorption (SEA), i.e. the energy absorption capability per unit mass, is higher than many metallic counterparts. However, the SEA of composite structures has been observed to decrease under dynamic crush loading when compared with quasi-static compression. This is different from metallic structures. For example, carbon fiber/vinyl ester composite crush tubes crushed at 2.0 m/sec were observed to have SEA of 23.8 J/gm, a decrease in SEA of 6.6 J/gm or 21.7% compared with quasi-statically loaded SEA of 30.4 J/gm.; Glass fiber/vinyl ester composite crush tubes were investigated with quasi-static compression and energy-absorbing modes were identified. The observed energy absorbing modes included tube corner splitting, composite delamination, matrix damage due to bending, and sliding friction of the composite with the plug type crush trigger. These same energy absorbing modes were observed in quasi-statically compressed and dynamically crashed carbon/vinyl ester composite crush tubes. Energy absorption attributable to corner splitting at quasi-static compression was estimated using standard tensile test results. Corner splitting was estimated to absorb less that 1% of the total energy absorbed by both the glass fiber composite and the carbon fiber composite crush tubes. Energy absorption attributable to delamination was estimated using the mode II (shear mode) strain energy release rate obtained using the end notch flexure (ENF) test. Under quasi-static compression, the glass fiber composite delamination SEA was found to be 1.31 J/gm or 6.4% of the total tube SEA. For carbon fiber composite crush tubes, the delamination SEA was found to be 0.84 J/gm or 2.8%f the total tube SEA.; Experiments seemed to suggest that sliding friction played an important role in the energy absorption of composite crush tubes. In an attempt to separate the sliding friction SEA from the SEA attributable to matrix damage due to bending, an innovative strip testing fixture was designed, fabricated, and tested and is presented in this thesis. Carbon fiber composite SEA, under quasi-static compression, attributable to matrix damage due to bending was found to be 18.9 J/gm or 62.2% of the total tube SEA. Under dynamic crush, the SEA attributable to matrix damage due to bending was found to be 18.6 J/gm or 78.1% of the total tube SEA. Carbon fiber composite SEA attributable to sliding friction under quasi-static compression was found to be 10.6 J/gm or 34.8% of the total tube SEA. Under dynamic crush, the SEA attributable to sliding friction was found to be 4.3 J/gm or 18.1% of the total SEA. The decrease in sliding friction SEA of 6.3 J/gm accounted for nearly all of the decrease in tube SEA of 6.6 J/gm between dynamic crush and quasi-static compression. Sliding friction was concluded to be responsible for the decrease in overall tube SEA from quasi-static compression to dynamic crush.